Mechanical ventilation is an important life support treatment of critically ill patients, and air pressure dynamics of human lung affect ventilation treatment effects. In this paper, in order to obtain the influences of seven key parameters of mechanical ventilation system on the pressure dynamics of human lung, firstly, mechanical ventilation system was considered as a pure pneumatic system, and then its mathematical model was set up. Furthermore, to verify the mathematical model, a prototype mechanical ventilation system of a lung simulator was proposed for experimental study. Last, simulation and experimental studies on the air flow dynamic of the mechanical ventilation system were done, and then the pressure dynamic characteristics of the mechanical system were obtained. The study can be referred to in the pulmonary diagnostics, treatment, and design of various medical devices or diagnostic systems.
As an important life-saving treatment, mechanical ventilation is adopted to ventilate patients who cannot breathe adequately on their own [
Nowadays, dynamic characteristics of respiratory system and models of different medical conditions are referred to in pulmonary diagnostics and treatments [
Because of the physical analogies between pneumatic and electrical systems, the structure of the human respiratory tract is usually presented as analogous to an electrical system [
To illustrate the pressure dynamic characteristics of PCV mechanical ventilation system, in this paper, first of all, mechanical ventilation system is considered as a pure pneumatic system, which has a better versatility and applicability, and may improve the effectiveness and accuracy of parameter identification.
Furthermore, based on the equivalent pneumatic system, a mathematical model of PCV mechanical ventilation system is set up, and influence of parameters on respiratory resistance
Moreover, to verify the mathematical model and void injury to real lung, a prototype PCV mechanical ventilation system of a lung simulator is proposed. On the basis of experimental and simulation study on the prototype system, its dynamic characteristic can be obtained and analyzed.
Last, influences of key parameters on the pressure dynamic characteristics are studied.
A typical simplified mechanical ventilation system, as shown in Figure
Structures of mechanical ventilation system and equivalent pneumatic system.
Mechanical ventilation system
Equivalent pneumatic system
According to the function of the ventilator, it can be regarded as an air compressor.
Efficiency of ventilation depends on the matching of ventilator settings to the actual mechanical properties of the respiratory system, which mainly consist of respiratory resistance
In this simplified system, the respiratory resistance
The respiratory compliance
Because the pressure of ventilation system is about 2 cm H2O~40 cm H2O, respiratory compliances
According to the working principle of mechanical ventilation system, its working process can be considered as inflation and deflation of a variable volume container. To facilitate research, the following assumptions are made [ air of the system follows all ideal gas laws; the temperature, pressure, and density field of air in the same capacity are uniform. At any time, state parameter of air anywhere in the capacity is the same; the dynamic process is quasi-balanced process; there is no air leakage during the working process; in each moment of dynamic process, the flow state of air is the same as the state of steady flow under the same conditions; the flow of air flowing into and out of the lung simulator is stable one-dimensional flow, equivalent to the flow of air through the nozzle contraction.
When air flows through throttle, its mass flow can be calculated by the equation when air flows through the LAVAL nozzle. When
In this study, air temperature is constant and equal to atmosphere temperature,
Volume flow of air can be calculated by the following equation:
The prototype ventilation system can be assumed as an isothermal system; the differential expression of Clapeyron equation
According to the definition of respiratory compliance
Then, the volume of the lung can be calculated by the following formula:
Based on the assumptions above, the resistance
The difference between the output pressure of the ventilator and the pressure in the lung is defined as the pressure loss
In the study, the pipe is plastic and its maximum diameter is 22 mm, and therefore according to [
According to (
In this study, to avoid injury to real lung, a lung simulator is adopted. The inlet diameter of the lung simulator is just 3.2 mm, and then it can be considered as a combined throttle of equivalent throttles 1 and 2, as shown in Figure
The experimental apparatus, shown in Figure
Configuration of experimental apparatus.
In this experiment, firstly, we open the ventilator and adjust the ventilator settings to the fixed value. When the ventilation system works steadily, we execute data acquisition and preservation.
Because the lung simulator is a passive lung simulator, the adopted model of ventilation is pressure controlled model (PCV). The values of the main ventilator settings, including inspiratory positive airway pressure (IPAP), expiratory positive airway pressure (EPAP), breaths per minute (BPM), inspiratory time
Values of the main ventilator setting.
Parameter | |||||
---|---|---|---|---|---|
IPAP (cmH2O) | EPAP (cmH2O) | BPM |
|
|
|
Value | 22 | 4 | 20 | 1 | 0.2 |
The fluctuation amplitude of air flow and pressure is so large and the frequency is so high that wavelet filter technology was adopted in this study [
Through the experiment, it can be calculated that the compliance
As the output dynamic of the ventilator is unassured and cannot be simulated exactly, in order to acquire precise simulation results, the output pressure of the ventilator is fitted, and the fitted output pressure is used as input pressure of tube, which connects to lung simulator.
In addition, the diameter (22 mm) of the tube is far greater than the inlet diameter (3.2 mm) of lung simulator; therefore, the respiratory resistance of the ventilation system mainly results from the resistance of the inlet of lung simulator, and the resistance due to the tube can be neglected.
The initial values of the parameter in simulation are the same as the values in experiment. The software, MATLAB/Simulink, is used for simulation.
The curve and fitted curve of output pressure of the ventilator as well as the curve of the air pressure in the lung simulator are shown in Figure
Curve and fitted curve of air pressure in tube.
Curve of air flow in the system.
Respiratory resistance of the system.
From Figure As the average IPAP and EPAP, in the report by the ventilator, are 21.3 cm H2O and 3.9 cm H2O, respectively, hence, the measured data are consistent with the ventilator report, and the experiment results are authentic and reliable. With a growth in the output pressure of the ventilator, the air pressure in the lung simulator rises. However, when the output pressure of the ventilator reaches the top flat, the air pressure in the lung simulator continues to rise, until it is equal to the output pressure of the ventilator. After that, the air pressure in the lung simulator declines with a decrease in the output pressure of the ventilator, until the EPAP. As can be seen, the air pressure in the lung simulator always lags behind the output pressure of the ventilator. The main reason is that the respiratory resistance and compliance block the increase in the air pressure in the lung simulator. It should be noticed that, if the inspiration time is set shorter, the respiratory resistance or compliance is big enough, and then the air pressure in the lung simulator may not reach IPAP.
As seen in Figure The simulation results have a good consistency with the experimental results, and this verifies the mathematical model above. In the inspiration process, with an increase in the output pressure of the ventilator, the input air flow of lung simulator rises sharply, but the rise velocity reduces continuously. When the output pressure of the ventilator gets to IPAP, the input air flow of lung simulator starts to decline. And finally the lung simulator stops inspiration when the air pressure in lung simulator is the same as the output pressure of the ventilator. In the expiration process, the output air flow of lung simulator increases sharply with a reduction in the output pressure of ventilator, and the rise velocity reduces constantly until the air pressure in lung simulator tends to be EPAP. When the output pressure of the ventilator sinks to EPAP, the output air flow of lung simulator starts to decline. And finally the output air flow tends to be zero when the air pressure in lung simulator tends to be EPAP. The main reasons for the difference between the experimental results and simulation results are the variation of the respiratory compliance and leakage of the ventilation system. In the simulation, the respiratory compliance is considered as a constant, and it is assumed there is no leakage in the ventilation system. However, the compliance of lung simulator varies with the pressure of lung simulator, and the leakage cannot be avoided in the experimental study.
As shown in Figure
The respiratory resistance fluctuates with time regularly; the fluctuation range of respiratory resistance value, during inspiration, is from 0.72 cm H2O/L/s to 3.58 cm H2O/L/s. During expiration, it is from 0.72 cm H2O/L/s to 3.98 cm H2O/L/s.
The variation of the respiratory resistance practically corresponds to the variation of the air mass flow of the lung simulator. However, when the air mass flow tends to zero, based on (
As the air pressure in human lung is very critical to mechanical ventilation treatment, and that is determined by the parameters of ventilation system, for the sake of a good treatment effect, it is necessary to study the influence of the parameters on the pressure dynamic of the ventilation system of the lung simulator.
According to the experimental study and simulation above, each parameter can be changed for comparison while all other parameters are kept constant, and the simulation results varying each parameter are illustrated in Figures
Influence of the IPAP on the air pressure dynamic.
Influence of the EPAP on the air pressure dynamic.
Influence of the BPM on the air pressure dynamic.
Influence of the
Influence of
Influence of
Influence of
Influence of the diameter
Influence of the diameter
As presented in Figure
As shown in Figure
As illustrated in Figure
As shown in Figure
From Figure
The respiratory compliance
Firstly, with the decline in the respiratory compliance
Furthermore, when the respiratory compliance
Finally, the amplitude of the respiratory resistance
As discussed above in Section
As shown in Figures
First of all, with a growth in the diameter
Furthermore, as shown in Figure
Lastly, the amplitude of the respiratory resistance
In this paper, the mechanical ventilation system was considered as a pure pneumatic system, and then a new mathematical model of mechanical ventilation system was set up. For the validation of the mathematical model, a prototype mechanical ventilation system of a lung simulator was proposed. Simulation and experimental studies on the air pressure dynamics of the lung simulator were done and the conclusions are summed up as follows. The measured data has a good consistency with the ventilator report, and the experiment is authentic and reliable. The simulation results are consistent with the experimental results, which verify the mathematical model. The air pressure in the lung simulator rises with a growth in the output pressure of the ventilator and declines with a decrease in the output pressure of the ventilator. The air pressure in the lung simulator always lags behind the output pressure of the ventilator. Increasing IPAP may lead to a distinct rise in maximum pressure of the lung simulator. The EPAP elevation may result in a significant rise in minimum pressure of the lung simulator. Influences of BPM, the inspiration time When the respiratory compliance When the diameter
The study can be referred to in the respiratory diagnostics, treatment, and design of various medical devices or diagnostic systems. In addition, it may accelerate research on the development of new diagnostic and treatments.
Effective area of throttle (
Critical pressure
Respiratory compliance (L/cm
Diameter of effective area (m)
Length (m)
Mass of air (kg)
Pressure (pa)
Air mass flow (kg/s)
Air volume flow (
Gas
Respiratory resistance (cm
Temperature (K)
Time (s)
Volume (
Friction coefficient
Density (kg/
Specific heat
Equivalent throttle 1
Equivalent throttle 2
Downstream side
Exhalation valve
Parameter of lung
Pressure loss
Upstream side
Tube.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The research is funded by Open Foundation of the State Key Laboratory of Fluid Power Transmission and Control.